The present invention relates to a semiconductor device and a method for fabricating the same, more specifically to a MIS (Metal Insulator Semiconductor) semiconductor device and a method for fabricating the same.
Many of the conventional p-MOS (p-channel Metal Oxide Semiconductor) transistors use as the gate electrodes an n-silicon layer, which facilitates their fabrication methods. P-MOS transistors using an n-silicon layer as the gate electrodes are called buried-channel p-MOS transistors because the channels are formed in the semiconductor substrates, spaced from the surfaces of the substrates.
However, it is known that the buried channel MOS transistor is generally more susceptible to the short channel effect in comparison with the surface channel MOS transistor. In applying such buried channel p-MOS transistor to a semiconductor device of high integration, such as an LSI or others, the channel region thereof is so short that even when a length the channel region even a little deviates due to troubles of fabrication precision, threshold voltage characteristics are much changed. Accordingly, for a semiconductor device of high integration, such as an LSI or others, the surface channel p-MOS transistor, whose threshold voltage characteristics little change with respect to a deviation of a length of the channel region, is required.
In the surface channel p-MOS transistor, for the channel to be formed near the surface of the semiconductor substrate it is necessary to use a p-silicon layer as the gate electrode. It is proposed that such p-silicon layer is formed by forming a silicon layer on a gate insulation film and then implanting boron (B) ions in the silicon layer.
However, boron atoms have a small mass, and, in ion implantation, pass through the silicon layer and the gate insulation film below the silicon layer to adversely arrive at the inside of the semiconductor substrate, with a result that a p-impurity is present in the channel region in the semiconductor substrate. Required characteristics cannot be obtained. To prevent the boron from arriving at the inside of the semiconductor substrate, boron must be implanted at low energy, but it is difficult in terms of characteristics of the ion implantation apparatus to control the ion implantation apparatus so as to implant boron ions at low energy.
Then, it is proposed to implant boron in the gate electrode by the use of decaborane (B.sub.10 H.sub.14) or boron fluoride (BF.sub.2). The method of implanting boron in the gate electrode by the use of boron fluoride is described in, e.g., Japanese Laid-Open Patent application No. 02-159069. In using decaborane or boron fluoride, these molecules have larger masses than boron, and can prevent the boron from arriving at the inside of the semiconductor substrate without much lowering implantation energy.
However it has been found that in the method of implanting decaborane in the gate electrode, hydrogen contained in decaborane bonds with the silicon of the gate insulation film, and the following heat treatment or others dissociates the hydrogen and the silicon to adversely increase dangling bonds. Often the hydrogen is dissociated from the silicon to adversely increase dangling bonds of the silicon in a case that a BT stress (Bias Temperature Stress) test is conducted with a plus or minus voltage applied to the gate and at a high ambient temperature. Increase of the dangling bonds increases an interface state density in the interface between the gate insulation film and the semiconductor substrate and also increases fixed charge in the gate insulation film, whereby device characteristics, such as drain current, threshold voltage, mutual conductance, etc. tend to be deteriorated, and accordingly reliability is lowered. This has been a disadvantage of the method of implanting decaborane.
Generation of dangling bonds by the influence of hydrogen is not limited to the implantation of decaborane into the gate electrode. Hydrogen is generated by film forming by, e.g., plasma CVD (plasma Chemical Vapor Deposition), plasma etching, etc., and the hydrogen has caused the same problem. In sintering for ohmic contact of electrodes, annealing is performed in an atmosphere of hydrogen and oxygen, and the hydrogen used here has caused the same problem.
In the method for implanting boron fluoride in the gate electrode, both fluorine and boron are present in the silicon layer, and the following heat treatment, etc. cause a diffusion speed of the fluorine and that of the boron to mutually influence on each other to cause mutually accelerating diffusion in which their diffusion speeds are increased. The boron passes through the gate insulation film to arrive at the channel region in the semiconductor substrate to cause deviations of a threshold voltage, etc. This has been a problem of the method of implanting boron fluoride. In addition, a large dose of boron fluoride causes the fluorine to adversely thicken the gate insulation film and resultantly to lower a threshold voltage.